For a magnetic recording medium, particularly a magnetic disk or a magnetic sheet which is suitable for high density recording and is required to have high durability, both surface smoothness and durability thereof are required to be improved in order to improve electromagnetic properties thereof. However, conventional methods have not yet been able to fully meet the above requirement.
That is, in order to realize high density recording of a magnetic disk, it is important that the surface of a magnetic layer is smooth and that ferromagnetic particles are uniformly dispersed in the magnetic layer. However, sufficient durability of a magnetic layer has not so far been obtained by the use of fine magnetic particles, by improvements of smoothing treatment such as calendering treatment and by techniques for dispersing magnetic coating composition. Further, the thickness of the magnetic layer is forced to become thinner and thinner as high density recording has become further developed.
Generally used magnetic layers contain carbon black having an average particle size of about 10 m.mu. as an antistatic agent. Surface electric resistance of a magnetic layer having from 3 to 8 m.mu. thickness is generally controlled so that it is about from 1.times.10.sup.6 to 1.times.10.sup.9 .OMEGA.. And when a magnetic layer is made thinner and the carbon black is used in the same content as before, the surface electric resistance becomes higher. In this case, in order to reduce the surface electric resistance, it is necessary that an amount of carbon black added are increased. It is known that since carbon black particles are finer than generally used pigments, carbon black is dispersed with difficulty, and further it deteriorates the dispersibility of magnetic particles and the surface properties of a coated surface of a magnetic recording medium. When carbon black is forcibly dispersed under improved dispersing conditions, the coated surface is made smoother, but durability of the magnetic layer decreases.
It is described in U.S. Pat. No. 3,630,910 that a fatty acid is used as the lubricating agent and that the total amount of the lubricating agent added is less than 7 wt%, with respect to the weight of the magnetic particles, in order to increase durability.
The amount of the lubricating agent is limited to less than 7 wt% because, when the lubricating agent is used in an amount of about 3 wt% or more, the magnetic layer of a video tape tends to become plasticized and experiences drop-outs, and when lubricating agents such as fatty acids and fatty acid esters are used, the magnetic layer tends to stick to the magnetic head in some cases. In the case of an audio tape, when a lubricating agent is used in an amount of about 6 wt% or more, the magnetic layer thereof tends to become plasticized, and tape squeal is easily generated by the influence of temperatures and humidities, .mu.m value of the magnetic layer decreases, an edge of a tape which was wound to a reel disorderly waves by rubbing with a flange in running and white powers (i.e., scratched powders) generate, thereby tending to stain the magnetic head. A magnetic recording medium, particularly a magnetic disk which has high commercial value, must have a high durability such that the magnetic disk can continuously rotate more than 10,000,000 times. In such a case, satisfactory high durability has not yet been obtained when conventional binder composition is used and a lubricating agent is used in an amount of less than 7 wt% per ferromagnetic particles.